1. Field of Invention
This invention relates to bearings that provide restraint of bearing elements such that translation and rotation between the bearing elements in directions substantially perpendicular to opposing bearing lands is permitted, commonly know as self-compensating hydrostatic bearings. The distance or bearing gap between a bearing land and opposing bearing guide surface is made nearly consistent and frictionless during motion in the allowed directions by a pressure or flow modulated fluid medium that occupies the bearing gap. The pressure of the fluid medium is varied relative to the instantaneous load on the bearing and resulting change in bearing gap so that a restoring force arises of sufficient magnitude to displace the bearing elements and restore the equilibrium bearing gap width. The invention specifically relates to an improved self-compensating hydrostatic bearing system.
2. Description of Prior Art
Self-compensating hydrostatic bearing designs as described in the prior art of U.S. Pat. Nos. 3,582,159 5,010,794 5,104,237 5,281,032 5,484,208 5,513,917 5,971,614 and 5,980,110 call for a flow restrictor or modulator to be located in a bearing carriage at the bearing land comprised of an elaborate arrangement of groves, holes and or annuluses. These features must be exact in location, position, dimension and surface finish for proper performance.
The bearing land adjacent to the flow restrictor is in proximity of, and opposed by, an adjacent surface on a rail or spindle within a mechanical assembly. In order for the flow restrictor to function, the nominal bearing gap as controlled dimensionally by the adjoining mechanical assembly, must be established to an accuracy of within thousands of an inch. This means that the components of the mechanical assembly must be machined to high accuracy and at great cost.
In most applications of hydrostatic bearings, two or more bearing lands having an integral pocket and restrictor will each oppose a different surface of a common mechanical element, spindle or rail. As a result, the flow modulator or restrictor located in each bearing carriage is nested within an assembly such that creating the proper nominal bearing gap at each bearing land requires high accuracy of construction between each bearing land, as well as between each utilized surface on the common bearing element, rail or spindle.
The flow restrictor is subject to fluid flow and has intricate shape and contours that render it very susceptible to clogging with impurities. In prior art, the flow restrictor is an integral part of the bearing carriage. Once the flow restrictor becomes clogged, the bearing design of prior art does not allow for ease of cleaning or replacement.
Prior art provides for bearings to operate in conjunctive pairs such that the regulating of delivery fluid flow and pressure to the bearing pocket in the bearing carriage is provided by the flow restrictor located in the second bearing carriage. The same relative relationship exists for the bearing pocket of the second bearing carriage and the restrictor of the first bearing carriage. In order for a bearing pair to bring rise to the proper restoring forces at each bearing pocket, great accuracy must be provided between the relative location of the bearing land adjacent to the bearing pocket and the bearing land adjacent to the restrictor in the bearing carriage as well as in the conjunctive bearing carriage.
The prior art requires that the flow restrictor reside in the bearing carriage. This imposes that the bearing carriage and the land be of sufficient size to contain both the bearing pocket and the flow restrictor.
Since the flow restrictor and bearing pocket are located in the bearing carriage of the prior art, very complex internal cross porting is utilized in order to create the required hydraulic communication between the pocket and the restrictor within the bearing carriage. The interconnecting porting must be machined into the bearing, as external piping is not possible. This construction makes for expensive manufacturing operations and bearing carriage parts that are not serviceable.
In the prior art, the individual flow restrictor is used to regulate the fluid provided to each bearing pocket. As a result each flow restrictor is provided with pressurized fluid from a remote or separate source. In order for the relative outputs of the flow restrictors to be correct, the relative difference in the remote or separate fluid sources must be minimized.
In the prior art of U.S. Pat. No. 5,064,297 a metal plate spring diaphragm controls flow by alternately deflecting and seating against adjacent throttling control valves. The plate spring diaphragm deflects according to the applied differential pressure. Since work must be done on the diaphragm by the differential fluid in order to deflect the diaphragm the response time of the diaphragm is increased. This delay in response time means reduces the accuracy of the bearing position.
In the prior art of U.S. Pat. No. 5,064,297 the deflection of the metal spring plate diaphragm brings about restoring forces that oppose the net force of the applied differential pressure. This condition can give rise to resonance and resulting inaccuracies of the bearing.
In the prior art of U.S. Pat. No. 5,064,297 large differential pressures can seat the diaphragm on one throttling valve. If this occurs the effective area which fluid pressure is applied is smaller on the side that lies against the throttling valve. The diaphragms position is fixed and fails to regulate fluid flow or bearing position.
Accordingly, several objects or advantages of the Shuttle Compensated Hydrostatic Bearing over hydrostatic bearings of prior art are as follows;
The Shuttle Compensated Hydrostatic Bearing includes a flow regulating valve or shuttle valve comprised of a shuttle and shuttle body which need not be integrated into a mating bearing surface but need only be connected by a fluid path. This means that the invention is more economical to manufacture because the prior art requires a precision flow restrictor consisting of a dimensionally precise combination of annuluses, holes and or slots machined into a bearing carriage with particular locational accuracy with respect to an opposing surface.
The shuttle valve of the current invention is a modular component whose function is dependent on hydraulic feedback and not position or relative location to an opposing rail surface or spindle surface. In the absence of these manufacturing constraints, the invention can be manufactured with great accuracy and low cost.
The invention compares fluid conditions of flow and pressure within the bearing gaps of pairs of bearings without dependence on the distance between the bearing lands of each bearing or respective rail surface or spindle surfaces. This feature of the invention allows for machining tolerances to be reduced within the assembly while providing a high positional accuracy and stiffness. The separable and modular configuration of the shuttle valve allows for ease of cleaning or replacement in case of clogging or the inadvertent introduction of contaminants. The bearing pockets in a bearing carriage of the invention hydraulically communicate with a shuttle valve instead of a restrictor mounted in a different bearing carriage. This configuration eliminates the need for strict dimensional relationship between restrictors as in prior art. The bearing carriage in the invention does not house the shuttle valve so that the length of the bearing carriage can be less than that of the prior art.
The shuttle valve is connected to the bearing carriage hydraulically without being located or embedded in the bearing land, as is typically the case in the prior art with the flow restrictor. This allows for a simplified hydraulic connection between the shuttle valve and the bearing carriage to be constructed of external tube or piping instead of elaborate internal cross-porting which is difficult and expensive to manufacture.
The shuttle valve of the current invention can provide pressure regulated fluid to a pair of bearing gaps while dividing the flow from a single source. This eliminates the need to maintain like inlet port pressure levels among separate fluid supplies and eliminates this as a source of error in the output pressures to the bearing gaps.
The shuttle of the current invention is not mechanically attached to the shuttle body. The position and performance of the shuttle is directly affected by only the applied differential fluid pressure. Since the movement of the shuttle is not encumbered by any mechanical restraint, the response time or lag of the shuttle to the instantaneous differential fluid pressure is minimized. As a result the positional accuracy of the bearing is kept high.
The shuttle of the current invention does not experience a mechanically induce restoring force when displaced from an equilibrium position. This means that a resonance or vibration of the shuttle due to opposing mechanical and hydraulic forces cannot exist. Therefore the current invention will not become unstable due to self-induced vibration.
At times of extreme fluid pressure differentials across the shuttle, the shuttle may seat against the shuttle body. In this condition a small area of the shuttle surface is prevented from exposure to the fluid pressure within the shuttle body. At the same time the reduced flow to the outlet port gives rise to a higher pressure than that of the fluid escaping more rapidly at the opposite outlet port. Thus the current invention allows that a higher pressure is applied to a substantially equal surface area on the shuttle enabling the shuttle to prevent sealing completely and stalling the shuttle valve causing the affected bearing gap to collapse.
Further objects and advantages of the Shuttle Compensated Hydrostatic Bearing will become apparent from consideration of the drawings and ensuing description.